U.S. patent application number 10/429944 was filed with the patent office on 2003-11-13 for solid polymer fuel cell and method of manufacturing the same.
This patent application is currently assigned to Mitsubishi Denki Kabushiki Kaisha. Invention is credited to Fukumoto, Hisatoshi, Hiroi, Osamu, Kurata, Tetsuyuki, Yoshida, Yasuhiro.
Application Number | 20030211380 10/429944 |
Document ID | / |
Family ID | 29405327 |
Filed Date | 2003-11-13 |
United States Patent
Application |
20030211380 |
Kind Code |
A1 |
Hiroi, Osamu ; et
al. |
November 13, 2003 |
Solid polymer fuel cell and method of manufacturing the same
Abstract
A solid polymer type fuel cell having a polyelectrolyte film
having a proton conductivity; an anode electrode and a cathode
electrode arranged on the opposite sides of the polyelectrolyte
film; and a gas flow channel for supplying gas to the both
electrodes, the anode electrode and the cathode electrode each
being composed of a catalyst layer that is in contact with the
polyelectrolyte film and a gas diffusion layer for allowing the
diffusion of gas supplied from the gas flow channel to the catalyst
layer, in which the gas diffusion layer included in the cathode
electrode is constructed of a carbon-containing material and the
surface of the carbon-containing material is modified to be
hydrophilic.
Inventors: |
Hiroi, Osamu; (Tokyo,
JP) ; Fukumoto, Hisatoshi; (Tokyo, JP) ;
Yoshida, Yasuhiro; (Tokyo, JP) ; Kurata,
Tetsuyuki; (Tokyo, JP) |
Correspondence
Address: |
LEYDIG VOIT & MAYER, LTD
700 THIRTEENTH ST. NW
SUITE 300
WASHINGTON
DC
20005-3960
US
|
Assignee: |
Mitsubishi Denki Kabushiki
Kaisha
Tokyo
JP
|
Family ID: |
29405327 |
Appl. No.: |
10/429944 |
Filed: |
May 6, 2003 |
Current U.S.
Class: |
429/480 ;
429/513; 429/532; 429/534; 429/535 |
Current CPC
Class: |
H01M 8/0234 20130101;
H01M 8/1007 20160201; H01M 8/1039 20130101; Y02P 70/50 20151101;
Y02E 60/50 20130101; H01M 8/1023 20130101; H01M 8/0245 20130101;
H01M 8/04156 20130101; H01M 8/0236 20130101 |
Class at
Publication: |
429/44 ;
429/30 |
International
Class: |
H01M 004/96; H01M
008/10; H01M 004/94 |
Foreign Application Data
Date |
Code |
Application Number |
May 10, 2002 |
JP |
2002-135831 |
Apr 8, 2003 |
JP |
2003-104282 |
Claims
What is claimed is:
1. A solid polymer type fuel cell comprising: a proton-conductive
polyelectrolyte film; an anode electrode and a cathode electrode
arranged on the opposite sides of the polyelectrolyte film; and a
gas flow channel for supplying gas to the anode electrode and the
cathode electrode, the anode electrode and the cathode electrode
each including a catalyst layer that is in contact with the
polyelectrolyte film and a gas diffusion layer for diffusing the
gas supplied from the gas flow channel to the catalyst layer,
wherein the gas diffusion layer included in the cathode electrode
is constructed of a carbon-containing material and the surface of
the carbon-containing material is modified to be hydrophilic.
2. A solid polymer type fuel cell according to claim 1, wherein the
carbon-containing material is made of carbon fibers.
3. A solid polymer type fuel cell according to claim 2, wherein the
surface of the carbon fibers is coated with a hydrophilic
material.
4. A solid polymer type fuel cell according to claim 3, wherein the
hydrophilic material is constructed of metal oxide.
5. A solid polymer type fuel cell according to claim 3, wherein the
coating of the hydrophilic material on the surface of the carbon
fibers is removed from a contact portion between the coating
portion and a material that constitutes the catalyst layer and/or a
material that constitutes the gas flow channel.
6. A method of manufacturing a solid polymer type fuel according to
claim 1, comprising forming an anode electrode and a cathode
electrode on the both sides of a proton-conductive polyelectrolyte
film and forming gas flow channels on the outsides of the anode
electrode and the cathode electrode, wherein the surface of the
carbon-containing material is modified to be hydrophilic and is
then used as a gas diffusion layer included in the cathode
electrode.
7. A method of manufacturing a solid polymer type fuel cell
according to claim 6, wherein the carbon-containing material is
made of carbon fibers and the surface of the carbon fibers is
coated with metal oxide precipitated from an aqueous solution that
contains metal fluoride by dipping the carbon fibers in the aqueous
solution that contains metal fluoride.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a solid polymer type fuel
cell and a method of manufacturing the same. In particular, the
present invention relates to a solid polymer type fuel cell capable
of keeping a polyelectrolyte film in a wet state and continuously
supplying gas into a catalyst layer in an efficient manner to allow
an increase in the efficiency of the cell and to cut operation
costs, and to a method of manufacturing the solid polymer type fuel
cell.
[0003] 2. Description of the Related Art
[0004] In recent years, clean power generation systems have been
demanded because of growing environmental awareness, and in
particular, attentions have been paid on fuel cells as one of these
systems. The fuel cells include phosphoric type fuel cells, molten
carbonate type fuel cells, solid electrolyte type fuel cells, solid
polymer fuel cells, and so on. Among them, the solid polymer type
fuel cells are being studied and developed actively as they are
advantageous for their low power-generating temperatures and in
terms of size reduction as compared with other types of fuel
cells.
[0005] FIG. 2 is a cross sectional diagram for illustrating an
example of the conventional solid polymer type fuel cell. In the
figure, the solid polymer type fuel cell 21 includes a
polyelectrolyte film 22 having a proton conductivity, an anode
electrode 23 and a cathode electrode 24 arranged on the opposite
sides of the polyelectrolyte film 22, and gas flow channels 25, 25'
for supplying gas to the both electrodes 23 and 24. The anode
electrode 23 includes a catalyst layer 231 that is in contact with
the polyelectrolyte film 22, and a gas diffusion layer 232 for
allowing the diffusion of gas supplied from the gas flow channel 25
to the catalyst layer 231. Likewise, the cathode electrode 24
includes a catalyst layer 241 that is in contact with the
polyelectrolyte film 22 and a gas diffusion layer 242 for allowing
the diffusion of gas supplied from the gas flow channel 25' to the
catalyst layer 241. Here, the gas flow channels 25 and 25' are
constructed by arranging plural grooved portions in separator
plates 26 and 26', respectively.
[0006] In such a solid polymer type fuel cell 21, fuel gas (e.g.,
hydrogen gas) is supplied to the anode electrode 23 and an
oxidizing agent (e.g., the air or oxygen gas) is supplied to the
cathode electrode 24. An external circuit (not shown) connects the
both electrodes to allow the fuel cell 21 to be actuated. More
specifically, the anode electrode 23 receives the supply of
hydrogen gas or the like from the gas flow channel 25 formed in the
separator 26 at first. Then, the hydrogen gas passes through the
gas diffusion layer 232 and diffuses toward the catalyst layer 231.
Subsequently, the hydrogen gas having reached the catalyst layer
231 generates protons and electrons by an oxidation reaction with a
catalyst. The protons pass through the solid polyelectrolyte film
22 and move to the cathode electrode 24. On the other hand,
electrons pass through the external circuit (not shown) to reach
the cathode electrode 24. In the cathode electrode 24, the protons
having passed through the solid polyelectrolyte film 22, the
electrons having been transferred from the external circuit, and
oxygen gas or the like to be supplied through the gas flow channel
25' formed in the separator plate 26' and the gas diffusion layer
242 are reacted with each other by the catalyst layer 241 and
converted into water. Concurrently, an electromotive force is
generated between the electrodes, so that it becomes possible to
obtain electric energies as outputs.
[0007] For effectively performing the above reaction without
interruption, it is important to decrease an ion-conduction
resistance and to supply gas to the catalyst layers 231 and 241 of
the respective electrodes 23 and 24. For decreasing the
ion-conduction resistance, the high polymer electrolyte may be
always kept in a wet state with water. On the other hand, such
water should be continuously drained because the contact between
gas and the catalytic layer 231 (241) is prevented when the water
generated in the cathode electrode 24 is retained on the surface of
the catalyst layer 231 (241) or such water closes holes in the gas
diffusion layer 232 (242).
[0008] For preventing the holes in the gas diffusion layer 232
(242) from being closed with water, water repellent finishing is
widely performed on an electrode material using a
fluorine-contained resin or the like. In particular, the gas
diffusion layer 232 (242) is provided as a supply channel for
allowing the gas supplied from gas flow channels 25 (25') to reach
the catalyst layer 231 (241) and is generally made water repellent.
However, even though the water repellent finishing avoids the
retention of water in the gas diffusion layer 232 (242), the water
is retained on the surface of the catalyst layer 232 (242) as the
transfer of water on the surface of the catalyst layer 232 (242) to
the gas diffusion layer 232 (242) is prevented. Therefore, it
becomes difficult to continuously supply the gas to the catalyst
layer 232 (242).
[0009] In the solid polymer type fuel cell, as described above, the
more the polyelectrolyte film contains water, the more the
ion-conduction resistance decreases to improve the performance of
the fuel cell. For this reason, the gas is supplied after being
heated by an external humidifier in advance to keep the
polyelectrolyte film in a wet state. When a liquid (i.e., water) is
vaporized for humidifying the gas, latent heat of vaporization can
be consumed as energies. Therefore, the more the degree of
humidification is increased for increasing the performance of the
fuel cell, the more the consumption energy increases. In addition,
there is another problem in that an increase in heat loss is caused
due to the heat dissipation from the body of a humidifier or heat
dissipation through a gas pipe arrangement from the humidifier to
the fuel cell.
[0010] For solving those problems drastically, there is a need to
operate the fuel cell at a smaller amount of gas humidification.
However, when a solid polymer type fuel having a general
configuration is operated at a low humidification area, the
performance of the fuel cell is significantly reduced as the water
content of the electrolytic film decreases. For operating the fuel
cell in a low humidification area, there is a need of designing the
fuel cell so as to keep the electrolytic film in a wet state. As
such a method, the following techniques have been known in the
art.
[0011] JP 7-326361 A discloses an electrode in which a
water-absorbing resin or a water-absorbing inorganic substance is
dispersed and mixed in a gas diffusion layer. In this prior art,
however, the water-absorbing substance is impregnated and dispersed
in holes of the gas dispersion layer. Therefore, there is a
disadvantage in that the gas-diffusing ability of the gas
dispersion layer is reduced as the void content thereof decreases.
When a water-absorbing resin is used, in particular, the resin is
swollen with water so that the void content is reduced. In
addition, an organic water-absorbing substance cannot be stable for
a long time under severe high-temperature and high-humidity
conditions in the fuel cell.
[0012] In addition, JP 11-45733 A discloses a solid polymer in
which hydrophilic inorganic fine particles such as silica or
alumina are coated on the gas diffusion layer together with carbon
particles and a hydrophilic layer is provided between a catalyst
layer and the gas diffusion layer. However, in this prior art,
water is retained on the surface of the catalyst layer as the
hydrophilic inorganic fine particles are arranged in the
neighborhood of the catalyst layer, even though a polyelectrolyte
film is prevented from being dried. Therefore, there is a problem
in that the continuous supply of gas to the catalyst layer is
difficult. Furthermore, the void content of the gas diffusion layer
decreases as the usage amount of the hydrophilic inorganic fine
particles increases. Therefore, a decrease in the gas-supplying
ability may occur. Furthermore, as the hydrophilic layer has a
small thickness, only a small humidification effect can be attained
by the vaporization of water being stored in this layer.
Furthermore, the invention of JP 11-45733 A is characterized in
that a thin layer of a hydrophilic substance is provided between a
catalyst layer and a dispersion layer. However, the space where gas
passes through is closed easily when water is accumulated in the
thin hydrophilic layer. Therefore, the so-called flooding
phenomenon becomes significant, resulting in a decrease in the
performance of the fuel cell.
[0013] Furthermore, the technique disclosed in JP 6-275282 A has
been known for keeping an electrolytic film in a wet state by the
application of a hydrophilic material in the inside of the catalyst
layer. However, as the catalyst layer is substantially thinner than
the dispersion layer, the amount of water being accumulated in the
catalyst layer is small. In addition, water is accumulated on the
surface of the catalyst layer when the catalyst layer itself is
hydrophilic. Therefore, the contact between the catalyst and the
gas become more difficult as the water covers the surface of the
catalytic layer. Such a state also represents the so-called
flooding phenomenon so that the performance of the fuel cell
decreases to a large extent.
[0014] In the case of operating the fuel cell under
low-humidification conditions, a portion on the upstream of a gas
flow in the cathode surface of the solid polymer type fuel cell
tends to be dried with low-humidified gas, while a portion on the
downstream of the gas flow tends to be humidified with
reaction-product water. Therefore, there is a problem in that, due
to such water distribution, it is impossible to utilize the surface
of the cathode in a uniform manner, resulting in a decrease in the
performance of the fuel cell.
SUMMARY OF THE INVENTION
[0015] Therefore, an object of the present invention is to provide
a solid polymer type fuel cell capable of keeping a polyelectrolyte
film in a wet state and continuously supplying gas into a catalyst
layer in an efficient manner to allow an increase in the efficiency
of the cell and to cut operation costs, and also to provide a
method of manufacturing the solid polymer type fuel cell.
[0016] The present invention is directed to a solid polymer type
fuel cell including: a proton-conductive polyelectrolyte film; an
anode electrode and a cathode electrode arranged on the opposite
sides of the polyelectrolyte film; and a gas flow channel for
supplying gas to the anode electrode and the cathode electrode, the
anode electrode and the cathode electrode each being composed of a
catalyst layer that is in contact with the polyelectrolyte film and
a gas diffusion layer for diffusing the gas supplied from the gas
flow channel to the catalyst layer, characterized in that the gas
diffusion layer included in the cathode electrode is constructed of
a carbon-containing material and the surface of the
carbon-containing material is modified to be hydrophilic.
[0017] Furthermore, the invention is directed to a method of
manufacturing a solid polymer type fuel. The method includes
forming an anode electrode and a cathode electrode on the both
sides of a proton-conductive polyelectrolyte film and forming gas
flow channels on the outsides of the anode electrode and the
cathode electrode, characterized in that the surface of the
carbon-containing material is modified to be hydrophilic and is
then used as a gas diffusion layer included in the cathode
electrode.
[0018] According to the above constitution, the polyelectrolyte
film can be kept in a wet state while continuously supplying gas to
the catalyst layer in an efficient manner, so that it becomes
possible to provide a solid polymer type fuel cell allowing an
increase in cell efficiency while preventing an increase in
operation costs.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] In the accompanying drawings:
[0020] FIG. 1 is a cross sectional view for illustrating an example
of a solid polymer type fuel cell in accordance with the present
invention;
[0021] FIG. 2 is a cross sectional view for illustrating an example
of a solid polymer type fuel cell; and
[0022] FIG. 3 is a graph that shows an output voltage with respect
to a cathode gas dew-point in each of fuel cells obtained in
Examples 1 to 6.
DETAILED DESCRPITION OF THE INVENTION
[0023] Hereinafter, the present invention will be described in more
detail with reference to the attached drawings.
[0024] Referring now to FIG. 1, there is shown a cross sectional
view for illustrating an example of a solid polymer type fuel cell
according to the present invention.
[0025] In FIG. 1, a solid polymer type fuel cell 11 includes a
proton-conductive polyelectrolyte film 12, an anode electrode 13
and a cathode electrode 14 arranged on the opposite sides of the
polyelectrolyte film 12, and gas flow channels 15 and 15' for
supplying gas to the both electrodes 13 and 14. The anode electrode
13 includes a catalyst layer 131 that is in contact with the
polyelectrolyte film 12, and a gas diffusion layer 132 for allowing
the diffusion of gas supplied from the gas flow channel 15 to the
catalyst layer 131. Likewise, the cathode electrode 14 includes a
catalyst layer 141 that is in contact with the polyelectrolyte film
12 and a gas, diffusion layer 142 for allowing the diffusion of gas
supplied from the gas flow channel 15' to the catalyst layer 141.
Here, the gas flow channels 15 and 15' are constructed by arranging
plural grooved portions in separator plates 16 and 16',
respectively. The configuration of the above-described fuel cell
itself-is similar to that of the conventional one. However, the
present invention is characterized in that the gas diffusion layer
142 included in the cathode electrode 14 is constructed of a
carbon-containing material and the surface of the carbon-containing
material is modified to be hydrophilic.
[0026] The polyelectrolyte film 12 is preferably a polyelectrolyte
film having high proton conductivity and high gas-barrier property
without any electron conductivity, which is stable under
environmental conditions inside the fuel cell. In general, a
polyelectrolyte film having a perfluoro main chain with a sulfonic
group may be used.
[0027] The catalyst layer 131 included in the anode electrode 13
may be, for example, an alloy of platinum and a noble metal (e.g.,
ruthenium, rhodium, or iridium), an alloy of platinum and a base
metal (e.g., vanadium, chromium, cobalt, or nickel), or the like
supported on carbon fine particles or the like.
[0028] The catalyst layer 141 included in the cathode electrode 14
maybe, for example, platinum supported on carbon black fine
particles, platinum black, or the like.
[0029] As described above, the present invention is characterized
in that the gas diffusion layer 142 included in the cathode
electrode 14 is constructed of a carbon-containing material and the
surface of the carbon-containing material is modified to be
hydrophilic. The catalyst layer 141 is in contact with the gas
diffusion layer 142, so that water on the surface of the catalyst
layer 141 moves quickly toward the gas diffusion layer 142 having a
high hydrophilic property. Consequently, it is possible to solve
the conventional problem in which the continuous supply of gas to
the catalyst layer becomes difficult as water is retained on the
surface of the catalyst layer. Furthermore, the carbon-containing
material to be used is preferably made of carbon fibers. In
particular, a porous material made of carbon fibers such as carbon
paper, carbon cloth, or carbon non-woven fabric. In addition, the
diameter of the carbon fiber is preferably in the range of 5 to 20
.mu.m, for example. Here, the term "hydrophilic property" used in
the present invention is represented by a contact angle of 0 to
10.degree. with respect to the surface of the solid.
[0030] The surface of a carbon-containing material, especially
carbon fibers is preferably covered with a hydrophilic material
having a thickness of 50 nm to 1 .mu.m. In addition, when the
material is comprised of carbon fibers with a diameter of 10 .mu.m,
the amount of the carbon fibers to be covered is preferably 2 to
15% by weight with respect to the weight of the carbon-containing
material. Here, the term "the surface of carbon fibers" means the
whole surface of the carbon fibers including the inner surface of a
porous material comprised of the carbon fibers.
[0031] In the case of using the carbon fibers covered with a
hydrophilic material, water that has moved to the gas diffusion
layer 142 spreads so as to cover the surface of each carbon fiber
having a continuous layer that is modified to be hydrophilic.
Consequently, the water surface area of the inside of the gas
diffusion layer 142 becomes extremely large, so that the vapor rate
thereof is increased. Furthermore, the movement of water from a
water-excess portion to a dry portion is accelerated. The gas fed
from gas flow channels 15' is humidified sufficiently by the
resulting water vapor in the course of reaching the catalyst layer
141. As a result, even if the gas under low-humidification
conditions is supplied to the fuel cell, the same performance as
that of the prior art can be maintained. Therefore, the energies
consumed by an external humidifier become smaller as compared with
the prior art. In addition, it becomes possible to lower the
temperature of pipe arrangement that supplies gas from the external
humidifier to the fuel cell, as compared with the prior art.
Therefore, the energy loss by heat dissipation decreases. In
addition, by operating the fuel cell under further
low-humidification conditions, the so-called flooding phenomenon,
which is a phenomenon in which the performance is reduced due to
retention of water in the electrode, becomes less liable to
occur.
[0032] The low-humidification driving condition described here
indicates an area in which the dew point of the supplied gas is
about 10.degree. C. or more lower than the temperature of the main
body of the fuel cell when the temperature of the main body of the
fuel cell is 75 to 80.degree. C., an area in which the dew point of
the supplied gas is almost 15.degree. C. or more lower than the
temperature of the main body of the fuel cell when the temperature
of the main body of the fuel cell is 70 to 75.degree. C., or an
area in which the dew point of the supplied gas is about 20.degree.
C. or more lower than the temperature of the main body of the fuel
cell when the temperature of the main body of the fuel cell is 60
to 70.degree. C.
[0033] In the present invention, the space through which gas passes
easily is not closed by water because the whole of the dispersion
layer, which occupies the largest space volume in the inside of the
solid polymer type fuel cell, is utilized. As described below, the
flooding may occur when the fuel cell is operated for a long time
in a high humidification area. However, the flooding does not occur
in the low humidification area.
[0034] In addition, the whole surface of the carbon-containing
material provided as a material of the gas diffusion layer is made
hydrophilic, so that the surface area of water in the dispersion
layer becomes extremely large. Therefore, a vapor rate is high so
that, when the gas under low-humidification conditions passes
through the dispersion layer, the gas can be efficiently
humidified. Consequently, it becomes possible to prevent a decrease
in content of the electrolytic film.
[0035] Furthermore, the hydrophilic layer on the surface of the gas
diffusion layer continuously extends over the whole surface of the
dispersion layer, so that water is able to move easily from a
water-excess portion to a water-deficient portion in the surface.
Therefore, a decrease in ion conductivity which occurs due to the
flooding resulting from a water excess or due to a water deficiency
is overcome so that the whole surface can be efficiently utilized.
Therefore, the output of the fuel cell can be increased.
[0036] The method for modifying the surface of the
carbon-containing material to be hydrophilic is not particularly
limited. Various kinds of methods well-known in the art can be
applied.
[0037] There is known a method for forming hydrophilic groups on
the surface of a carbon-containing material by performing plasma,
corona, and anodizing treatments thereon. In such a method,
however, when a porous material is used as such a carbon-containing
material, it is difficult to make the inside thereof be rendered
sufficiently hydrophilic and to keep the hydrophilic property of
the resulting product for a long period of time.
[0038] For avoiding such disadvantages, the present inventors have
made extensive studies and found a material and a method which
allow the inside of the material to be imparted with sufficient
hydrophilic property and allow the hydrophilic property to be kept
for a long period of time even in the case of using a porous
material.
[0039] That is, the above-mentioned material may be a metal oxide,
and in particular, titanium oxide (TiO.sub.2), aluminum oxide
(Al.sub.2O.sub.3), and silicon dioxide (SiO.sub.2) are
preferable.
[0040] The above-mentioned method may be a CVD method for obtaining
a thin film from a gas phase, a sol-gel method for obtaining a
metal oxide by hydrolyzing metal alkoxide, a method for thermally
decomposing an organic metal complex, or the like. Among them, the
so-called liquid phase deposition (LPD) method for precipitating an
oxide thin film from a metal fluoride aqueous solution is
preferable. More specifically, the carbon-containing material
(e.g., carbon fibers) is dipped into a metal fluoride-containing
aqueous solution. Then, the surface of carbon fibers is coated with
a metal oxide which is precipitated from the metal
fluoride-containing aqueous solution.
[0041] The liquid phase deposition is particularly advantageous in
that a large-sized material of the gas diffusion layer can be
treated in large quantity, it is possible to make a uniform coating
on the material even though a porous material having a complicated
shape and comprised of carbon fibers is used, and that the
treatment costs or the like are low. In addition, as the coating
can be performed at a temperature near a normal temperature, only a
small amount of the treatment energy is required.
[0042] The above sol-gel process includes immersing the material of
the gas diffusion layer into a solution that contains
metal-alkoxide, drying the layer, and baking it at about
500.degree. C. This method also forms a coating of metal oxide. The
sol-gel process is capable of obtaining a high-purity metal oxide
film such as SiO.sub.2, Al.sub.2O.sub.3, or TIO.sub.2.
[0043] In addition, the thickness of the coating of a hydrophilic
material is preferably small for preventing a decrease in the
volume of holes of the gas diffusion film 142 and also for the
reason that a thin coating does not easily exfoliate. However, the
effects of modification on hydrophilic properties may not be easily
expressed due to a coating defect of the coating is too thin.
[0044] If the hydrophilic material to be applied is a metal oxide,
there is a possibility of an increase in resistance as the material
is a nonconductive material. In this case, it is effective to form
a conductive portion on a place obtained by slightly grinding the
materials that constitute the gas diffusion layer 142 and the
catalyst layer 141, and/or the material that forms the gas flow
channels, i.e., by slightly grinding a contact portion with the
separator plate.
[0045] The solid polymer type fuel cell 11 of the present invention
can be prepared by forming the anode electrode 13 and the cathode
electrode 14 on the opposite sides of the polyelectrolyte film 12.
Here, the anode electrode 13 includes the catalyst layer 131 and
the gas diffusion layer 132 and the cathode electrode 14 includes
the catalyst layer 141 and the gas diffusion layer 142.
Furthermore, on the outside of the both electrodes 13 and 14, the
gas flow channels 15 and 15' are formed, respectively. In addition,
the catalyst layers 131 and 141 may be prepared by a method in
which they are formed on a polyelectrolyte film 12, a method of
forming them on one side of each of the gas distribution layers 132
and 142, and a method of forming them as independent layers.
[0046] Here, a carbon plate is generally used for each of the
separator plates 16 and 16'.
[0047] A fuel cell constructed as described is incorporated in a
battery jig in the same manner as in the prior art. The generation
of electricity is performed by supplying humidified hydrogen or the
like into the anode electrode, while supplying oxidizer gas
including humidified air into the cathode electrode.
EXAMPLES
[0048] Hereinafter, the present invention will be described in more
detail with the following examples and comparative examples.
Example 1
[0049] (Hydrophilic Modification Treatment on the Surface of a
Carbon-Containing Material)
[0050] Carbon paper (TGP-H-090, manufactured by Toray Industries.
Inc.) was dipped into an aqueous solution that contains 0.1 moles/1
of ammonium titanium hexafluoride and 0.2 moles/1 of boric acid.
After defoaming, the solution was kept at 30.degree. C. for 20
hours to make the carbon paper a gas diffusion layer. The carbon
paper after the treatment was covered with a thin film of titanium
oxide, so that the carbon paper presented a color interfered with
the thin film of titanium oxide. From the difference between the
weights of carbon paper before and after the treatment, it was
found that 1.5 mg of titanium oxide per cm.sup.3 of carbon paper
was coated. The volume of the coating of titanium oxide was about
0.38.times.10.sup.-3 cm.sup.3 equivalent to the specific gravity of
the coating, so that the volume of hole in the carbon paper was
hardly reduced. Therefore, the coating treatment does not directly
prevent-the gas diffusion.
[0051] Next, the carbon paper before the treatment and the carbon
paper after the treatment were dipped into pure water for 10
seconds, followed by comparing their respective water absorption
amounts by a gravimetric method. After the treatment, the carbon
paper after the treatment retains about 10 times of water per unit
area, as compared with the carbon paper before the treatment.
Therefore, it was found that the hydrophilic property of the carbon
paper was significantly increased by the coating. The coated carbon
paper was slightly ground using a waterproof abrasive paper (#2000)
to remove a part of the covering layer on the surface to be in
contact with the separator plate and the catalyst layer.
[0052] (The Formation of Catalyst Layer)
[0053] The catalyst used in this example was a catalytic metal
supported on the carbon black (acetylene black). A cathode catalyst
was one supporting 50% by weight of platinum and an anode catalyst
was one supporting 50% by weight platinum-ruthenium metal.
[0054] In 1 part by weight of catalyst powders, 5 parts by weight
of a perfluoro-polyelectrolyte (9 parts by weight) solution (FSS-1,
manufactured by Asahi Glass Co., Ltd.) and 1 parts by weight of
water were added, followed by mixing with stirring to obtain
uniform paste. Then, the catalyst paste was screen-printed on a
25-.mu.m PET (polyethylene terephthalate) film and then dried.
Then, a polyelectrolyte film (50 .mu.m in thickness, Aciplex film
manufactured by Asahi Kasei Co., Ltd.) was sandwiched between the
films having the above catalytic layer and was then subjected to a
hot press at 150.degree. C. for 2 minutes to remove the PET film to
thereby form a catalyst layer on the polyelectrolyte film. The
catalyst layer was formed into a square shape of 50 mm.times.50
mm.
[0055] (The Formation of Cell}
[0056] A polyelectrolyte film having the catalyst layer described
above is sandwiched between the gas diffusion layers. Furthermore,
they were sandwiched between carbon plates having gas flow grooves
to provide a solid polymer type fuel cell as shown in FIG. 1. As
gas diffusion layers, carbon paper treated with the hydrophilic
modification was used on the cathode electrode side, while carbon
paper without hydrophilic modification treatment was placed on the
anode electrode side.
[0057] (Operation of Cell)
[0058] The fuel cell received the supply of hydrogen gas on its
anode electrode side and the supply of the air at a normal pressure
on the cathode electrode side. In addition, their flows were
adjusted such that the utilization of hydrogen gas was 70% and the
utilization of oxygen on the air side was 40%. The gas was supplied
to the cell after humidifying with an external humidifier. In
addition, the temperature of the cell was adjusted to 80.degree. C.
Regarding the humidity of the supply gas, the external humidifier
was adjusted such that the anode side was a dew point of 65.degree.
C. and the cathode side was a predetermined dew point. Then, the
cell was operated at a current density of 300 mA/cm.sup.2 and an
output voltage at 24 hours after the initiation was measured. The
changes of voltages and resistances of the fuel cell with respect
to the humidifying temperature were shown in Table 1.
Comparative Example 1
[0059] A fuel cell was prepared and operated in the same manner as
in Example 1, except that carbon paper without hydrophilic
modification treatment was used for the gas diffusion layer on the
cathode electrode side. The changes of voltages and resistances of
the fuel cell with respect to the humidifying temperature were
shown in Table 1.
Comparative Example 2
[0060] A fuel cell was prepared and operated in the same manner as
in Example 1, except of the hydrophilic modification treatment to
be performed by the following steps.
[0061] (Hydrophilic Modification Treatment)
[0062] In 1 part by weight of titanium oxide powders in average
particle size of 0.5 .mu.m, 3 parts by weight of a
perfluoro-polyelectrolyte (9 parts by weight) solution (FSS-1,
manufactured by Asahi Glass Co., Ltd.) and 3 parts by weight of
water were added, followed by mixing with stirring to obtain
uniform paste. Then, the paste was dried after screen printing on
one side of the carbon black. Subsequently, the carbon paper before
the treatment and the carbon paper after the treatment were dipped
into pure water for 10 seconds, followed by comparing their
respective water absorption amounts by a gravimetric method. After
the treatment, the carbon paper after the treatment retains about 3
times of water per unit area, compared with the carbon paper before
the treatment. Therefore, it was found that the hydrophilic
property of the carbon paper was significantly increased by the
coating of titanium oxide particles. The resulting hydrophilic
layer was arranged so as to be in contact with the catalytic
layer.
Example 2
[0063] A fuel cell was prepared and operated in the same manner as
in Example 1, except that the treatment was performed for 5 hours.
It was found that a coating amount of 0.2 mg per cm.sup.3 of the
carbon paper was obtained from the difference between the weights
of the carbon paper before and after the treatment.
[0064] When a comparison was made between the water absorption
amount of the carbon paper before the treatment and that after the
treatment in the same manner as in Embodiment 1, the carbon paper
after the treatment retained about 3 times of water per unit area
as compared with the carbon paper before the treatment.
[0065] The changes of voltages and resistances of the fuel cell
with respect to the humidifying temperature were shown in Table
1.
Example 3
[0066] A fuel cell was prepared and operated in the same manner as
in Example 1, except that the treatment was performed for 40 hours.
It was found that a coating amount of 2.5 mg per cm.sup.3 of the
carbon paper was obtained from the difference between the weights
of the carbon paper before and after the treatment.
[0067] When a comparison was made between the water absorption
amount of the carbon paper before the treatment and that after the
treatment in the same manner as in Embodiment 1, the carbon paper
after the treatment retained about 10 times of water per unit area
as compared with the carbon paper before the treatment.
[0068] The changes of voltages and resistances of the fuel cell
with respect to the humidifying temperature were shown in Table
1.
Example 4
[0069] A fuel cell was prepared and operated in the same manner as
in Example 1, except that the hydrophilic modification treatment is
performed by the following steps.
[0070] The changes of voltages and resistances of the fuel cell
with respect to the humidifying temperature were shown in Table 1.
Further, the resistances of the fuel cell with respect to the
humidifying temperature were shown in Table 1.
[0071] The changes of voltages and resistances of the fuel cell
with respect to the humidifying temperature were shown in Table
1.
[0072] (Hydrophilic Modification Treatment)
[0073] Silica gel was dissolved as much as possible in a
hydrosiliconfluoric acid 2 mol/1 solution. An aqueous solution was
obtained by dissolving boric acid in this solution to have a
concentration of 0.024 mol/1. The carbon paper was dipped in the
aqueous solution, which was then held at 30.degree. C. for 20 hours
to coat the carbon paper with a silica thin film.
[0074] It was found that the carbon paper was coated with 0.9
mg/cm.sup.3 of silica. Next, the carbon paper before the treatment
and the carbon paper after the treatment were dipped into pure
water for 10 seconds, followed by comparing their respective water
absorption amounts by a gravimetric method. After the treatment,
the carbon paper after the treatment retains about 8 times of water
per unit area, as compared with the carbon paper before the
treatment. Therefore, it was found that the hydrophilic property of
the carbon paper was significantly increased by the coating. The
coated carbon paper was slightly ground using a waterproof abrasive
paper (#2000) to remove a part of the covering layer on the surface
to be in contact with the separator plate and the catalyst
layer.
[0075] The changes of voltages and resistances of the fuel cell
with respect to the humidifying temperature were shown in Table
1.
Example 5
[0076] A fuel cell was prepared and operated in the same manner as
in Example 1, except that the hydrophilic modification treatment is
performed by the following steps.
[0077] The changes of voltages and resistances of the fuel cell
with respect to the humidifying temperature were shown in Table
1.
[0078] (Hydrophilic Modification Treatment)
[0079] Carbon paper was dipped into a solution prepared by adding
0.5 parts by weight of diethanol amine and 50 parts by weight of
isopropanol into 1 part by weight of titanium tetraisopropoxide and
was then pulled out of the solution, followed by drying at
100.degree. C. for 10 minutes. Subsequently, the carbon paper was
subjected to heating at 300.degree. C. for 1 hour to remove organic
components to coat a thin film of titanium oxide. At the time of
heating treatment, a part of the titanium oxide thin film was
peeled and dropped off. It was found that the carbon paper was
coated with 3 mg/cm.sup.3 of titanium oxide. Next, the carbon paper
before the treatment and the carbon paper after the treatment were
dipped into pure water for 10 seconds, followed by comparing their
respective water absorption amounts. After the treatment, the
carbon paper after the treatment retains about 6 times of water per
unit area, compared with the carbon paper before the treatment.
Therefore, it was found that the hydrophilic property of the carbon
paper was significantly increased by the coating. The coated carbon
paper was slightly ground using a waterproof abrasive paper (#2000)
to remove a part of the covering layer on the surface to be in
contact with the separator plate and the catalyst layer.
Example 6
[0080] (Hydrophilic Modification Treatment on the Surface of a
Carbon-Containing Material)
[0081] The hydrophilic modification treatment was performed in the
same manner as in Example 1. (The Formation of Catalyst Layer)
[0082] The catalyst used in this example was a catalytic metal
supported on the carbon black. A cathode catalyst was one
supporting 50% by weight of platinum and an anode catalyst was one
supporting 50% by weight of platinum-ruthenium metal.
[0083] In 1 part by weight of cathode catalyst powders, 5 parts by
weight of a perfluoro-polyelectrolyte (9 parts by weight) solution
(FSS-1, manufactured by Asahi Glass Co., Ltd.) and 1 parts by
weight of water were added, followed by mixing with stirring to
obtain uniform paste for the cathode.
[0084] In 1 part by weight of anode catalyst powders, 7 parts by
weight of a perfluoro-polyelectrolyte (9 parts by weight) solution
(FSS-1, manufactured by Asahi Glass Co., Ltd.) and 1 parts by
weight of water were added, followed by mixing with stirring to
obtain uniform paste for the anode. Then, those catalyst pastes
were screen-printed on a 25-.mu.m PET film and then dried to obtain
a transcription catalyst layer. Then, a polyelectrolyte film (50
.mu.m in thickness, Aciplex film manufactured by Asahi Kasei Co.,
Ltd.) was sandwiched between the films having the above catalytic
layer and was then subjected to a hot press at 150.degree. C. for 2
minutes to transfer the catalyst layer on to the polyelectrolyte
film. The catalyst layer was formed into a square shape of 50
mm.times.50 mm.
[0085] (Formation of Cell}
[0086] The cell was prepared in the same manner as in Example
1.
[0087] (Operation of Cell)
[0088] The fuel cell received the supply of hydrogen gas on its
anode side and the supply of the air at a normal pressure on the
cathode side. In addition, their flows were adjusted such that the
utilization of hydrogen gas was 80% and the utilization of oxygen
on the air side was 50%. The gas was supplied to the cell after
humidifying with an external humidifier. In addition, the
temperature of the cell was adjusted to 75.degree. C. Regarding the
humidity of the supply gas, the external humidifier was adjusted
such that the anode side was a dew point of 65.degree. C. and the
cathode side was a predetermined dew point. Then, the cell was
operated at a current density of 250 mA/cm.sup.2 and an output
voltage at 24 hours after the initiation was measured. The changes
of voltages and resistances of the fuel cell with respect to the
humidifying temperature were shown in Table 1.
[0089] Furthermore, in FIG. 3, the output voltages for the dew
point of the cathode gas in each fuel cell obtained in Examples 1
to 6 were shown.
1 Voltage (mV) Resistance (m.) Dew point of cathode gas Example
60.degree. C. 65.degree. C. 70.degree. C. 60.degree. C. 65.degree.
C. 70.degree. C. Example 1 678 697 710 5.8 5.1 4.3 Example 2 661
670 705 6.1 5.3 4.5 Example 3 679 695 708 5.8 5.0 4.3 Example 4 665
686 705 6.0 5.4 4.5 Example 5 651 670 701 6.1 5.3 4.6 Example 6 711
712 717 6.2 5.16 4.1 Comparative 605 655 705 8.0 6.6 5.0 Example 1
Comparative 631 661 694 6.5 5.7 4.6 Example 2
[0090] Table 1 shows that the fuel cell of Example 1 has a
resistance smaller than that of the fuel cell of Comparative
Example 2. Particularly, in a lower humidification area where the
dew point of air is 65.degree. C. or less, the resistance is
dramatically decreased as a result of the effect for modifying the
surface of the gas diffusion layer to be hydrophilic. Therefore,
according to the present invention, the gas diffusion layer has a
humidifying effect to improve the performance of the fuel cell with
a decrease in the resistance of the polyelectrolyte film. Such an
effect becomes significant in the low humidification area. From
Table 1, it is found that the fuel cell of Example 1 has a higher
voltage and higher performance than the fuel cell of the
Comparative Example 1 or Example 2 as the surface of the material
that constitutes the gas diffusion layer is coated with a metal
oxide having a high hydrophilic property. Particularly, in the low
humidifying area where the dew point of the air which is gas to be
supplied to the cathode electrode is 65.degree. C. or less, an
excellent effect of modifying the hydrophilicity can be found,
contributing the improvement of the performance of the fuel
cell.
[0091] As is evident from Table 1, the fuel cell of Example 2 has a
shorter hydrophilic modification treatment time of the diffusion
layer, as compared with Example 1. Therefore, the surface of the
carbon paper is not sufficiently covered with a hydrophilic film.
Therefore, it can be considered that the performance improving
effect is small in the low humidifying area.
[0092] As is evident from Table 1, the fuel cell of Example 3 has a
longer hydrophilic treatment time and a large amount of the coating
in comparison with those of Example 1, while the amount of water
absorbed in the carbon paper and the performance of the fuel cell
are equal to those of Example 1. In other words, even though the
thickness of the coating is higher than that of Example 1, the
effect of improving the performance of the fuel cell is not
changed.
[0093] As is evident from Table 1, the gas diffusion layer can be
modified to be hydrophilic while the same effect can be obtained
even though silica is used as metal oxide as described in Example
4.
[0094] As is evident from Table 1, just as in the case of Example
5, the adaptation of sol-gel method also allows the hydrophilic
modification of the gas diffusion layer and the same effect can be
obtained.
[0095] As is evident from Table 1, the fuel cell of Example 6 is
designed such that the configuration of the catalyst layer and the
operational conditions of Example 1 are modified. Example 6 shows
an output voltage under low-humidification operational conditions,
which is higher than that of Example 1.
* * * * *